Add test for duplicate data when resaving model to fits

When resaving a model to fits, asdf loses track of array-hdu
links and saves arrays in both hdu and as internal blocks in the
asdf tree. This can make an asdf tree that is larger than what
can be stored in a fits hdu.

See asdf issue: asdf-format/asdf#1232

This test is skipped as it fails with the current asdf version.

modify imports and use context manager for test_fits

Co-authored-by: James Davies <jdavies@mpia.de>

use context manager in test_fits

Co-authored-by: James Davies <jdavies@mpia.de>

improve test docstring in test_fits

jwst/datamodels/tests/test_fits.py

@@ -4,7 +4,8 @@
 import numpy as np
 import pytest
 
-from jwst.datamodels import JwstDataModel, RampModel
+from jwst import datamodels
+from jwst.datamodels import ImageModel, JwstDataModel, RampModel
 
 
 @pytest.fixture
@@ -68,3 +69,26 @@ def test_casting():
         total = dm.data.sum()
         dm.data = dm.data + 2
         assert dm.data.sum() > total
+
+
+@pytest.mark.skip(reason='fix required for asdf')
+def test_resave_duplication_bug(tmp_path):
+    """
+    An issue in asdf (https://github.com/asdf-format/asdf/issues/1232)
+    resulted in duplication of data when a model was read from and then
+    written to a fits file. A resave results in array data being written to
+    both hdus and as internal blocks within the asdf tree (which is then
+    stored in the asdf hdu).
+    """
+    fn1 = tmp_path / "test1.fits"
+    fn2 = tmp_path / "test2.fits"
+
+    arr = np.zeros((1000, 100), dtype='f4')
+    m = ImageModel(arr)
+    m.save(fn1)
+
+    with datamodels.open(fn1) as m2:
+        m2.save(fn2)
+
+    with fits.open(fn1) as ff1, fits.open(fn2) as ff2:
+        assert ff1['ASDF'].size == ff2['ASDF'].size